The present invention relates to a process for producing reduced iron and thermal cracking of heavy oils or, more particularly, relates to a process which is a combination of the thermal cracking of heavy oils to produce lighter oils and cracked gases simultaneously along with the manufacture of reduced iron by the reduction of an iron ore utilizing the products obtained in the above mentioned thermal cracking as the reducing agent.
As is well known, it is an unavoidable current or future trend in the petroleum industry that the crude oils supplied to the industry become heavier and heavier along with the decreasing availability of high-quality lighter crude oils due to the exhaustion of the petroleum resources. Accordingly, early establishment of the technology for producing lighter oils, e.g. gasoline and gas oil, by the thermal cracking of such heavier crude oils is a matter of great concern for those in a diversity of fields as it is indispensable for the future development of the industry.
One of the processes of early establishment for the production of lighter oils by the thermal cracking of heavy oils is the so-called fluidized-bed catalytic cracking (FCC) process in which a heavy oil is contacted with catalyst particles of silica-alumina and the like in a fluidized state and catalytically cracked. A problem in the FCC process is the necessity of frequent regeneration of the catalyst since the activity of the catalyst decreases relatively rapidly due to the deposition of the carbon or coke formed in the thermal cracking of the heavy oil on the catalyst particles. Moreover when residual oil is used as feed stock, metals such as Ni and V are deposited on catalyst surfaces, and thus regeneration of this catalyst with the metal deposit is very difficult. Another disadvantage of the FCC process is the limitation in the starting heavy oils since the process is usually applicable only to distilled oils such as gas oils and several kinds of high-grade residual oils among heavy oils.
In connection with the above described FCC process, the so-called fluid coking process is also widely practiced in which the by-product coke formed in the thermal cracking of the heavy oil is taken out as a product. The principle of this method is the thermal cracking of the heavy oil with the powdery coke in the fluidized state as the medium for heat transfer as well as the fluidizing medium. Therefore, this process involves no problem of deactivation of the catalyst by the deposition of the by-product coke thereon because the powdery coke is used not as a catalyst but merely as the medium for the heat transfer and fluidization and is advantageous in the ease of processing heavy oils to be used generally for the production of feed oils to the FCC process. The by-product coke is discharged out of the reactor and a part thereof is used by combustion as the heat source for pre-heating the powdery coke circulating in the reactor, the balance of the by-product coke being obtained as a product. In contrast to the delayed coking process as a method for processing heavy oils, this fluid coking process is advantageous in that the process can be operated as a completely continuous process and that the yields of the cracking products are high. The process is, however, defective in the quality of the coke as the product because the only use of the product coke is as a fuel.
On the other hand, a process recently under rapid development in the iron and steel making industry is the production of so-called reduced iron by the direct reduction of iron ore in a solid state brought into contact with a reducing agent. Usually the iron ore is reduced to the reduced iron which is further melted and refined in an electric furnace into a steel. In comparison with the conventional steel making process using a blast furnace and a converter in which the iron ore is first reduced in a blast furnace into a pig iron containing an excess amount of carbon which is then subjected to the oxidizing removal of the excess of carbon, together with the removal of the accompanying silicon, phosphorus and other impurities in a converter to give a sound steel, this process of direct reduction has several advantages that the process does not involve excessive reduction of the ore followed by oxidation thereof and that the process does not require any coking coal such as the coke material used in the blast furnace. Nevertheless, the process has found no world-wide prevalence because hydrogen and carbon monoxide as the reducing agents used in the process are available with sufficient economy only in several limited regions throughout the world.
To give a more detailed explanation for the above mentioned direct reduction iron making process by means of gaseous reductants, the iron oxides in the iron ore are reduced with the gaseous reducing agent composed of hydrogen and carbon monoxide obtained by contacting natural gas, i.e. methane, with an oxidizing gas, e.g. steam or carbon dioxide, at a high temperature in the presence of a catalyst. That is, the reaction involved in this process is a solid-gas contacting reaction between iron oxides and a reducing gas irrespective of the type of the furnace for the reduction which may be a furnace using a fluidized bed or fixed bed or a shaft furnace.
As is mentioned above, this process is very promising as a technology to be developed in future owing to the above described advantages but, on the contrary, includes several problems in the availability of the energy source and difficulties in the operational conditions:
(a) The most economical source as the reducing agent is natural gas at least for the moment and the regions where natural gas is available with economy are limited in the world.
(b) Reforming of the natural gas into the reducing gas, i.e. a gaseous mixture which is principally composed of carbon monoxide and hydrogen, is not an inexpensive process due to the large investment for the installation of the plant and the large running cost because the facilities for the production of the reducing gas, i.e. reformer, must be constructed using tubes of expensive heat-resisting steel in large numbers filled with a large amount of the catalyst.
(c) In contrast to the blast furnace process in which the maximum temperature in the furnace eventually exceeds 1500.degree. C., the reaction temperature in the direct reduction iron making process usually can not exceed 850.degree. C. notwithstanding the desirable higher productivity and higher efficiency of energy obtained at a higher temperature of the reducing gas. This is because, when operated at an excessively high temperature, the particles of the metallic iron formed by the reduction adhere to each other eventually resulting in the phenomenon of sintering with the layer of the particles forming a blocked continuous body so that the process can run no longer with stability. When the process is performed using a fluidized bed, in particular, in which the particle size distribution of the iron ore is finer than otherwise, increase of the reducing temperature higher than 800.degree. C. cannot be expected due to sintering of fine partially reduced iron particles, and the advantages of the higher velocities of the reducing reaction and heat transfer inherently obtained in a fluidized bed process are restricted.
In view of the above described current trend in the exhausting situation of petroleum resources and the development of the iron making technology, it is an object of this invention to establish a process in which a by-product of the thermal cracking of heavy oils is utilized as the reducing agent in the production of reduced iron concurrently with the production of lighter oils.